I ran into a problem with NGINX failing to start on boot/reboot on my Debian 8 (Jessie) server. After reviewing what seemed like a hundred sites to try to find a fix, I stumbled across one solution that worked, but was incredibly inelegant. This was to add:
RestartSec=30s Restart=on-failure to nginx.service in the [Service] section using the override.conf. It worked but didn't fix the underlying problem.
A quick look using `journalctl -u nginx` showed that the service was failing because the IPv6 address hadn't been assigned to the network adaptor yet.